The use of terbium and cerium co-activated green-emitting phosphors in mercury vapor discharge lamps for fluorescent lighting applications is well established. The most commonly used of these phosphors include LaPO4:Ce,Tb, (Ce,Tb)MgAl11O19, and (Gd,Ce,Tb)MgB5O10 (referred to herein as CBT). These green-emitting phosphors are typically mixed with a red-emitting phosphor such as Y2O3:Eu and a blue-emitting phosphor such as BaMgAl10O17:Eu or Sr5(PO4)3:Cl,Eu to form a blend which emits with an overall white color when excited by the 254 nm radiation generated by the mercury vapor discharge.
U.S. Pat. No. 4,319,161 describes phosphors with the general composition (Y,La)1-x-y-zCexGdyTbz(Mg,Zn)1-pMnpB5O10. The method of producing these pentaborate phosphors involves dry mixing oxides of the rare earth elements, the oxide or hydrated carbonate-hydroxide of magnesium, manganese carbonate, the oxide of zinc and boric acid, and then subjecting the mixture to two or three firings in a weakly reducing atmosphere. In U.S. Pat. No. 6,085,971, Tews et al. describes that improved brightness, processing, and stability under the influence of short-wave UV radiation may be achieved in luminescent metaborate phosphors of the formula (Y,La)1-x-y-zCexGdyTbz(Mg,Zn,Cd)1-pMnpB5-q-s(Al, Ga)q(X)sO10, in which X is Si, Ge, P, Zr, V, Nb, Ta, W, or a combination thereof. Still, the synthesis method involved firing at two temperatures, often with a comminution step in between.
Predictably, methods that require repeated grinding and firing steps are labor intensive which generally means higher manufacturing costs. Furthermore, the use of large quantities of volatile boric acid leads to furnace contamination and generates a significant waste stream.
Unlike the above methods, U.S. Pat. Nos. 4,719,033 and 5,068,055 describe a single-step firing process for making europium-activated strontium tetraborate, SrB4O7:Eu, a UVA-emitting phosphor. The process involves adding a SrCO3/Eu2O3 mixture to an H3BO3 slurry at >90° C. to form a (Sr,Eu)B6O10.5H2O precipitate along with excess SrCO3/Eu2O3 in a 2:1 ratio. The precipitate is then fired to yield the SrB4O7:Eu phosphor. No boric acid is used in the firing step. The hydrated precipitate is fired after drying without adding additional compounds.
Thus, it would be an advantage to have a simpler process for making a CBT phosphor.